Lambda's recombination system, Red, works well in the absence of the host strand-invasion protein RecA, but only if the phage DNA is allowed to replicate. In the absence of RecA, Red appears to require double chain breaks at different locations in the two participating parents. These features suggests that in the absence of RecA protein, Red-mediated recombination proceeds by annealing of exposed complementary chains. The chains are presumably exposed by the action of lambda's exonuclease and annealed by the action of lambda's beta protein. This and other features of lambda Red recombination will be tested. E. coli's RecF pathway operates on lambda with features reminiscent of the Red pathway. Mutant derivatives of that pathway have altered properties. The RecF pathway and those derivatives will be examined with regard to the roles of double-chain breaks and of DNA replication in the recombination process. Yeast recombines by a pathway that shows strong similarities to lambda's Red pathway, as postulated by Szostak, Orr-Weaver, Rothstein and Stahl. Recombination is dependent on double chain cuts, and the exposure of 3' overhangs indicates the action of ana enzyme analagous to lambda's exonuclease. We will test other features of the model of Szostak et al. as well as other features of recombination in yeast.